Inexperienced swimmers often tend to hold their breath while they swim, releasing the contents of their lungs in a burst just before rotating their head and body to inhale. This results in several negative outcomes for the swimmer.

First, the length of time that is available for taking in oxygen (during the recovery phase) is determined by the swimmers stroke rate. If part of that time is spent expelling air from the lungs, then the time for taking in air is reduced. The only way for the swimmer to get a full inhale is to slow their stroke on the breathing side resulting in an uneven stroke.

Second, one of the best ways to remain relaxed while swimming is to maintain a long exhale. (Try it now: inhale deeply, then let out a long, steady exhale. The point of relaxation comes at the end of the exhale, right?) The longer the athlete swims, the more stress they will accumulate.

Third, by holding their breathe, the swimmer begins to rely on the buoyancy of their lungs to remain close to the surface of the water as opposed to using their stroke and kick to accomplish this. As the athlete continues to swim, their body position in the water will begin to sag as the volume of air being inhaled decreases.

I would propose that an alternative to this is to focus on the breathing and to maintain an uneven count where the time of exhalation is slightly longer than the inhalation. Example: while swimming freestyle and using a breath-every-three pattern, exhale continuously while your face is in the water, counting each stroke as one beat. This will result in a three out/one in pattern. A drill that can be used to practice this is to kick on your side for 6 beats(or kicks) then rotate to the other side for 6 beats. While doing this, have your face up for the first two beats, then turn your face to the bottom of the pool for four beats and exhale continuously until you rotate your body on the next count of six.

I find that this technique also works well for cycling and running. Focus on your breathing and practice exhaling smoothly for a count of four and inhaling strongly for a count of three. (Or if you are running harder, use 3-out/2-in instead.) See if this helps keep you relaxed and breathing deeper during your next hard interval.